From what I understand, something like a Retroleum Smart card in diagnostic mode would be able to check the ram chips, I can't check this myself because my TC2048 doesn't boot up for some reason.
Unless you can desolder the chips and test them, relying upon the compatibility with a standard Spectrum rom to run one of the Spectrum diagnostic roms in a expansion would be your best bet.
My TC may have a faulty rom, it uses a 27128 not a 23 series. I've tested my ram chips and they work.